The present invention relates to desmodromic and cam systems for internal combustion engines with intake and exhaust valves.
Most conventional internal combustion piston driven engines utilize valve trains to induct an air/fuel mixture into the cylinders and to expel the burned air/fuel mixture from the cylinders. Typically, each cylinder is assigned at least one intake valve and at least one exhaust valve. The valves are pushed down by rockers thereby opening the valve and pushed upwardly by springs thereby closing the valve. When the valve stem is pushed down by the rocker to open the valve, the spring is compressed. The valve is closed when the spring decompresses thereby pulling the valve stem up through the valve guide until the head of the valve is seated in the valve seat.
For example, in a typical four-stroke engine, an intake valve is opened by an intake rocker which receives an input force from an intake cam lobe while the piston goes down inducting an air/fuel mixture into the cylinder. This is known as the induction stroke. While the intake valve stem is being pushed down through an intake valve guide, an intake spring concentrically positioned around the intake valve stem is compressed. Next, the cam lobe continues to rotate allowing the intake spring to decompress. The intake spring pushes the intake valve back up through the intake valve guide until the intake valve is seated in the intake valve seat. The piston also moves back up the cylinder. At this point in the combustion process, the air/fuel mixture is compressed. This stage is known as the compression stroke. With both the intake and exhaust valves closed so that the combustion chamber is sealed tight, a spark is then produced by a spark plug which ignites the air/fuel mixture wherein the rapidly expanding hot gasses force the piston downward with great energy creating power. This is known as the power stroke. The exhaust valve is then opened by an exhaust rocker receiving input from an exhaust cam lobe. The piston moves up the cylinder and the exhaust valve expels the burned air/fuel mixture, also known as the exhaust stroke. The exhaust cam lobe continues to rotate and allows an exhaust spring to push the exhaust valve back to the closed position.
The aforementioned conventionally configured valve train system for opening and closing the valves have proven to be highly effective and reliable in the past. However, closing the valve by the force of the spring does have some disadvantages. For example, pushing the valve open against the force of the spring consumes engine power. The springs are strong such that the valves will close in accordance with the profile of the cam lobe and before the cam lobe pushes the rocker to reopen the valve during its next cycle. The valve springs are continuously pushing the valves closed and work must be performed to overcome such spring tension wasting energy that could be used to create output power. Another disadvantage is that because the cam mechanism cannot afford to have any “bounce” from the springs, the cam profile has to be somewhat gentle, i.e., it must gently push the valve, but never shove it. This means the valve must open slowly like a water faucet—not quickly like a light switch, for example. Another disadvantage is that when the motor is turned at high rpms, the valves can “float” and hit the piston. In other words, the spring does not traverse the valve back to the closed position fast enough such that the piston hits the valve. Valve float happens when the speed of the engine is too great for the valve spring to handle. As a result, the valves may stay open and/or “bounce” on their seats.
To overcome these disadvantages, innovative desmodromic valve trains have evolved over about the last century; however, in a very slow technological pace and in most applications with limited success. The term “desmodromic” arises from the two greek words: “desmos” (controlled or linked), and “dromos” (course or track). A desmodromic system is also known as a system that provides “positive valve actuation” wherein both strokes are “controlled.” The desmodromic valves are those which are positively closed by leverage system or follower, rather than relying on the more conventional springs to close the valves.
Desmodromic valve trains have several advantages over conventional spring closed valve trains. A first major advantage is that in a desmodromic valve system, there is no wasted energy in driving the valve train. The reason is that the constant force of the springs in a conventional spring closed valve train is removed.